Considerable effort has been expended in recent years to develop improved heterogeneous catalysts for the oxidation of volatile carbon compounds and for the reduction of nitrogen oxides to nitrogen. Such efforts have been directed not only toward the development of more effective catalysts for use in the manufacture of organic chemicals and for the reduction of atmospheric pollution by industrial processes involving the manufacture and use of nitric acid, but also toward the reduction of atmospheric pollution by exhaust gases from internal combustion engines.
Among the catalytic compositions which have been proposed for reducing the concentration of nitrogen oxides in off-gases for nitric acid plants and exhaust gases of internal combustion engines are such platinum group metals as platinum, palladium, rhodium, and ruthenium and the oxides of metals from the first transition series of the Periodic Table such as iron, cobalt, and nickel and of rare earth metals such as lanthanum, neodymium, and praseodymium. Certain catalytic compositions, including perovskites, have been proposed by Mai et al, in U.S. Pat. Nos. 3,900,428 and 3,901,828 and by Kobylinski et al, in U.S. Pat. No. 3,907,968.
Although some of the proposed catalysts are better than others, all of them have certain weaknesses. For example, the effectiveness of platinum group metals in oxidation reactions is lessened by exposure to elevated temperatures. Other proposed catalysts are effective only at high temperatures that require catalyst supports and enclosures made of materials which are scarce and difficult to fabricate. Some of the proposed catalysts for the reduction of nitrogen oxides promote the formation of undesirably large amounts of ammonia instead of nitrogen when the reducing agent is hydrogen. Others promote the formation of undesirably large amounts of intermediate oxidation products in the oxidation of hydrocarbons instead of promoting complete oxidation to carbon dioxide and water. Other catalysts, including the platinum metals and some of the transition and rare earth metal oxides, lose their catalytic activity upon exposure to alternate oxidizing and reducing environments from industrial processes and internal combustion engines operating under frequently changing conditions.
Still other proposed catalysts have reduced catalytic activity after exposure to normally nonreactive components of gas mixtures. For example, the transition and rare earth metal oxides have reduced activity as catalysts for the oxidation of carbon monoxide and hydrocarbons in the presence of water. Platinum metal catalysts lose their catalytic activity upon exposure to internal combustion engine exhaust gases containing compounds of lead, sulfur, phosphorus, chlorine and other materials derived from additives conventionally employed in automotive fuels and lubricants. Thus, there is a need for stable catalysts which are low in cost, simple to prepare, and selective in promoting desired oxidation-reduction reactions at relatively low temperatures.